1. Field of the Invention
This invention relates to the measurement of the concentrations of copper ions and formaldehyde in an electroless copper plating bath. More specifically, it relates to the measurement of the formaldehyde concentration spectrophotometrically and the use of a reagent in the measurement procedure which is reactive with formaldehyde but is not reactive with other components of the bath.
2. Description of the Prior Art
Electroless copper plating processes have been widely used for plating a metal film onto non-conductive plastics and for making printed circuit boards. The process usually utilizes a bath consisting of a strongly basic aqueous solution of a copper salt such as copper sulfate, a complexing agent such as N,N,N,N-tetrakis(2-hydroxypropyl) ethylenediamine, and a reducing agent, such as formaldehyde. Experience has shown that in the bath the copper ion and formaldehyde concentrations should be held at a specific ratio for optimum plating rates, plating thickness and bath stability, and replenishment of the copper ions and formaldehyde should be based on separate measurements of the two concentrations. A desirable means for measuring the formaldehyde concentration would be with a spectrophotometer. Methods have been proposed for spectrophotometrically measuring the formaldehyde concentration in a solution by reacting a reagent and the formaldehyde to produce a reaction product whose concentration is measurable and indicative of the formaldehyde concentration. Most of these methods would prove unsuitable for measuring formaldehyde in an electroless copper plating bath because they use reagents which would produce a strong reaction and react not only with the formaldehyde but also with the other components in the bath with the result that the formaldehyde concentration measurement would be affected. Another method has been proposed in an article "The Colorimetric Estimation of Formaldehyde by Means of the Hantzsch Reaction", T. Nash, Biochemical Journal, 55, 416, London (1953), in which the reagent chosen reacts only with formaldehyde. However, this method has been suggested for use only to measure formaldehyde concentration in living bacterial suspensions and in a biological environment and has never been suggested for use in measuring formaldehyde concentration in an electroless copper plating bath.